I don't have time to help you much, but consider that there are no net external forces on the particles when the nucleus disintegrates. So the net momentum of the system (which is 0 kgm/s) is conserved.

Now, momentum is a vector, so the momentum components in the x and y directions are also conserved. So set the initial momentum component in the x direction (0) equal to the sum of the final momentum components of the pieces. Do the same for the y direction.

-Dan